<form method="get" id="searchform" action="<?php bloginfo('url'); ?>">
  <label class="hidden" for="s"><?php _e(''); ?></label>
  <input class="form-control input-sm" type="text" size="20" required="required" placeholder="Search..." name="s" style="width:160px" />
  <input class="btn btn-primary btn-sm" type="submit" id="searchsubmit" value="搜索" />
</form>